Optical interconnect apparatus

ABSTRACT

Embodiment of present invention provide an optical interconnect apparatus. The apparatus includes an optical signal path; a first set of pigtail fibers attached to a first end of the optical signal path via a first wavelength-division-multiplexing (WDM) filter; and a second set of pigtail fibers attached to a second end of the optical signal path via a second WDM filter. Embodiment of present invention further provide an interconnected optical system that includes a first optical transport terminal having a first set of optical signal ports and a second optical transport terminal having a second set of optical signal ports, with the two sets of optical signal ports being interconnected by the optical interconnect apparatus.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related to U.S. patent application titled“Optical Interconnect Apparatus and System”, having a filing date ofNov. 27, 2017 and a Ser. No. 15/732,559.

FIELD OF THE INVENTION

The present invention relates generally to structure of optical deviceand configuration of optical system in optical signal transportationand, in particular, relates to an apparatus for optical signalinterconnect.

BACKGROUND

Wavelength-division-multiplexing (WDM) is a key enabling technology intoday's high speed digital communication infrastructure that supportsvast amount of data transportation which in turn is essential for manydata centric informational applications such as, for example, manyinternet based applications. The transportation of vast amount of dataare made via optical digital signals over an extensive optical fibernetwork across the country or global and such optical signals, riding ondifferent wavelengths (or “colors”), are distributed and/orre-distributed among various parts or branches of the optical networkvia signal exchanges located at various data centers and otherfacilities. Sometimes, such signal distribution and/or re-distributionmay also involve conversion of signals from optical to electric, andthen from electric to optical, in terms of its carrying media.

When optical signals are distributed and/or re-distributed within anoptical network, it often involves interconnecting optical signals fromone signal handling unit, which may be, for example, an optical signaltransponder installed in a shelf hosted by a rack (“bay”), to anothersignal handling unit which may be located in a same room, in a differentroom of a same floor, or sometimes in a different floor. FIG. 10 is ademonstrative illustration of an interconnected optical system 900 as isknown in the art. System 900 is simplistically illustrated to include afirst bay 910 and a second bay 920 located, for example, in a same roomof a building. Bay 910 may include, from top to bottom, multiple shelfssuch as shelf 911, with each of the shelfs having multiple opticalsignal transponders.

An optical signal coming from an optical signal transponder located inbay 910 may be connected to another optical signal transponder locatedin bay 920 via a piece of fiber 902, which may have connectors 901 and903 at its two ends connecting to the signal transponders. Multipleoptical signals from bay 910 may need to be connected to multipledestinations in bay 920, and vise versus, using multiple pieces offibers. Generally the number of fibers needed equals to the number ofoptical signals being interconnected between the bays, which isdemonstratively illustrated in FIG. 10 by a second piece of fiber 904and the “dots” in between which represents the existence of many morefibers between bay 910 and bay 920.

With the ever increasing data rate, in particular rapid deployment ofWDM technology, the number of optical signals of different wavelengthsthat need to be interconnected between different bays, and sometimesbetween different shelfs in a same bay, has increased dramaticallyresulting in the explosive use of fibers in signal interconnection. FIG.11 is an exemplary picture of a traditional optical system 990 whereoptical signal interconnect among the bays are provided by an enormousstack of individual pieces of fibers 991 which, like cooked spaghetti,are usually “dumped” at the backside of the various bays.

SUMMARY

It becomes apparent to the applicants of present invention that, with aninterconnected optical system like the one 990 shown in FIG. 11, fibermanagement is a major concern when it comes to, among others, performingsystem maintenance, reliability assurance of signal traffic, andtroubleshooting when, for example, there is a fiber cut.

Embodiments of present invention provide an optical interconnectapparatus. The apparatus includes an optical signal path; a first set ofpigtail fibers attached to a first end of the optical signal path via afirst wavelength-division-multiplexing (WDM) filter; and a second set ofpigtail fibers attached to a second end of the optical signal path via asecond WDM filter.

In one embodiment, the optical signal path is a single continuousoptical fiber cable from the first end to the second end. In anotherembodiment, the optical signal path includes at least two pigtail fiberscoming of the first and second WDM filters respectively, with the twopigtail fibers being connectorized at their other respective ends, andconnected together through an optical adaptor thereby forming theoptical signal path. In yet another embodiment, the optical signal pathincludes three or more optical fibers, each of the three or more opticalfibers being connectorized and connected in a series to form the opticalsignal path.

In one embodiment, at least one of the first and second ends of theoptical signal path is connected to one of the corresponding first andsecond WDM filters through an optical adaptor. In another embodiment, atleast one of the first and second ends of the optical signal path ispigtailed from one of the corresponding first and second WDM filters.

In one embodiment, the first set of pigtail fibers includes at least 4individual pigtail fibers that are adapted to accommodate four opticalsignals of four different wavelengths respectively. In anotherembodiment, the first set of pigtail fibers includes at least 8individual pigtail fibers. In yet another embodiment, the first set ofpigtail fibers includes more than 8 individual fibers. The first andsecond set of pigtail fibers includes the same number of pigtail fibers.

In one embodiment, the first set of pigtail fibers are stacked togetherand mounted on one side of the first WDM filter. In another embodiment,the first set of pigtail fibers and the first end of the optical signalpath are at a same side of the first WFM filter.

According to one embodiment, an optical signal of a first wavelengthpropagating from a corresponding first pigtail fiber via the first WDMfilter to the optical signal path experiences less than 0.5 dB totalinsertion loss. According to another embodiment, the first WDM filterhas a cross-sectional area that is less than ten times a size of thefirst set of pigtail fibers being stacked together, in a directionvertical to the stacking.

Embodiment of present invention provide an interconnected opticalsystem. The system includes a first optical transport terminal having afirst set of optical transponders with a first set of correspondingoptical signal ports; a second optical transport terminal having asecond set of optical transponders with a second set of correspondingoptical signal ports; and an optical signal path connecting the firstset of optical signal ports with the second set of optical signal ports,wherein the first set of optical signal ports are connected to a firstend of the optical signal path via a first act of pigtail fibersattached to a first wavelength-division-multiplexing (WDM) filter, andthe second set of optical signal ports are connected to a second end ofthe optical signal path via a second set of pigtail fibers attached to asecond WDM filter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be understood and appreciated more fully fromthe following detailed description of embodiments of the invention,taken in conjunction with accompanying drawings of which:

FIG. 1 is a demonstrative illustration of an optical interconnectapparatus according to one embodiment of present invention;

FIG. 2 is a demonstrative illustration of an interconnected opticalsystem where optical signal interconnect is provided by opticalinterconnect apparatus according to embodiment of presented invention;

FIGS. 3-7 are demonstrative illustrations of optical assemblies orinterconnect kits with different input/output interfaces according tovarious embodiment of present invention;

FIG. 8 is a sample example of a WDM filter that maybe used in an opticalinterconnect apparatus or optical assembly illustrated in FIGS. 3-7;

FIG. 9 is a simplified cross-sectional view of a WDM filter showing itscross-sectional dimension relative to that of a set of pigtail fibersattached thereto;

FIG. 10 is a demonstrative illustration of an interconnected opticalsystem where optical signal interconnect among various optical terminalsare provided by multiple individual fibers as is known in the art; and

FIG. 11 is a picture of a typical operational optical system whereinterconnection is provided by a stack of individual fibers at thebackside of the bays.

It will be appreciated that for simplicity and clarity purpose, elementsshown in the drawings have not necessarily been drawn to scale. Further,in various functional block diagrams, two connected devices and/orelements may not necessarily be illustrated to be connected, forexample, by a continuous solid line or dashed line but rather sometimesa small gap between two lines extended from the two devices and/orelements may be inserted intentionally in order to illustrate theindividual devices and/or elements even though their connection isimplied. In some other instances, grouping of certain elements in afunctional block diagram may be solely for the purpose of descriptionand may not necessarily imply that they are in a single physical entityor they are embodied in a single physical entity.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a demonstrative illustration of an optical interconnectapparatus according to one embodiment of present invention. Morespecifically, optical interconnect apparatus 10 may include an opticalsignal path 15, a first set of pigtail fibers 11 coming out of a firstWDM filter 12 that is attached to a first end 13 of optical signal path15. Optical interconnect apparatus 10 may additionally include a secondset of pigtail fibers 19 coming out of a second WDM filter 18 that isattached to a second end 17 of optical signal path 15.

Throughout this application and unless specific stated otherwise, theterm “fiber” is used to refer to a fiber cable that, in addition to a“bare” fiber that normally includes only a fiber core and one or morelayers of cladding around the core, generally also includes someprotection jacket surrounding the “bare” fiber for strength and ease ofhandling of the fiber. Similarly, the term “pigtail fiber” is used torefer to a fiber cable terminated at or pigtailed to another device orgenerally object, unless specifically stated otherwise.

First set of pigtail fibers 11 may include multiple pigtail fibers, allterminated at or pigtailed to first WDM filter 12. In one embodiment,one or more of the other respective ends of the set of pigtail fibersmay be connectorized to include any suitable type of connectors such asLC, SC, FC, ST, or MT-RJ connectors. Typically, the number of pigtailfibers may range from 2 to 8, preferably from 4 to 8, althoughembodiment of present invention are not limited in this aspect and anumber larger than 8, in terms the number of pigtail fibers, is fullycontemplated as well and is considered fully within the spirit ofpresent invention.

Each pigtail fiber is capable of carrying an optical signal of adistinct wavelength, with the wavelength being arranged or designatedaccording to industry standard. All of the optical signals coming fromfirst set of pigtail fibers 11 are combined inside first WDM filter 12into a composite WDM optical signal and subsequently coupled intooptical signal path 15 via the first end 13 thereof. Being abidirectional optical device, a composite WDM optical signal coming fromoptical signal path 15, in a reverse direction, may be launched intofirst WDM filter 12 via the first end 13, and be subsequently divided orseparated into multiple optical signals of distinct wavelengths, witheach being coupled into its corresponding pigtail fibers by first WDMfilter 12. In a further embodiment, the composite WDM optical signal mayinclude a plurality of single wavelength optical signals traveling inboth directions. An optical signal of a distinct wavelength propagatingfrom a corresponding pigtail fiber of first set of pigtail fibers 11 viafirst WDM filter 12 to optical signal path 15 may experience less than0.5 dB total insertion loss, and in one embodiment less than 0.2 dBtotal insertion loss.

In one embodiment, optical signal path 15 maybe a single piece ofoptical fiber capable of carrying WDM optical signals. In anotherembodiment, optical signal path 15 may be two or more optical fibersconnected in series via various connectors. The fiber or fibers makingup optical signal path 15 are fiber cables and not “bare” fibers. One orboth of the first end 13 and the second end 17 of optical signal path 15may be pigtailed to first WDM filter 12 and/or second WDM filter 18.However, embodiment of present invention are not limited in this aspect.For example, one or both ends of optical signal path 15 may beconnected, instead of pigtailed, to first WDM filter 12 and/or secondWDM filter 18 via suitable connectors including LC, SC, FC, ST, or MT-RJtype connectors.

Second set of pigtail fibers 19, similar to first set of pigtail fibers11, maybe terminated at or pigtailed to second WDM filter 18 and mayinclude a number of pigtail fibers that equals to the number of pigtailfibers in first set of pigtail fibers 11. Moreover, second set ofpigtail fibers 19 may be capable of carrying a set of optical signals ofdistinct wavelengths, decided by second WDM filter 18, which correspondsto the set of optical signals that may be carried by first set ofpigtail fibers 11, decided by first WDM filter 12. In other words, firstWDM filter 12 and second WDM filter 18 preferably have matchingwavelength-dependent functionalities. Similar to first set of pigtailfibers 11, second set of pigtail fibers 19 may be connectorized to haveconnectors at their respective ends, and the pigtail fibers are pigtailfiber cables.

FIG. 2 is a demonstrative illustration of an interconnected opticalsystem where optical interconnect is provided by one or more opticalinterconnect apparatus according to embodiment of presented invention.More specifically, in FIG. 2 optical system 200 is demonstrativelyillustrated to include a first bay 210 and a second bay 220 that may belocated, for example, in a same room, in different rooms or in differentfloors. Bay 210 may include multiple shelfs, such as shelf 211, each ofwhich may include multiple optical transponders, such as transponder211.1. Several optical signals, sometimes a significant number ofoptical signals, coming from several optical transponders located in bay210 may be connected to their corresponding optical transponders locatedin bay 220 via an optical interconnect apparatus 20. Opticalinterconnect apparatus 20 may be an optical interconnect apparatus 10 asbeing demonstratively illustrated in FIG. 1 according to one embodimentof present invention. However, embodiment of present invention are notlimited in this aspect and other optical interconnect apparatus oroptical assemblies or interconnect kits, such as those demonstrativelyillustrated in FIGS. 3-7 below, may be used as well, and may be used inconnection with an optical signal path.

Using optical interconnect apparatus 10 illustrated in FIG. 1 as anexample for the below description of FIG. 2, without losing the genericnature and essential spirit of present invention, multiple opticaltransponders in bay 210, in a same shelf or different shelfs, may beconnected to first set of pigtail fibers 11 of optical interconnectapparatus 10. With first set of pigtail fibers 11, optical interconnectapparatus 10 may be able to accommodate, for example, 4 to 8 or evenmore number of optical transponders. Similarly at the second end ofoptical signal path 15, a similar (4 to 8 or even more) number ofcorresponding optical transponders may be connected to second set ofpigtail fibers 19 of optical interconnect apparatus 10.

During operation, multiple optical signals of different wavelengths maybe coupled through connectors at the end of first set of pigtail fibers11 into first WDM filter 12, which subsequently combines the multipleoptical signals into a single composite WDM optical signal. The combinedcomposite WDM signal may be coupled to second WDM filter 18, throughoptical signal path 15, and be subsequently divided into individualoptical signals of their original distinct wavelengths. The signals arethen distributed via second set of pigtail fibers 19 and connectors atthe ends thereof to their corresponding optical transponders at bay 220.According to one embodiment, optical signal interconnect between thefirst and second bay 210 and 220 may be bidirectional. In other words,optical signals may propagate from first bay 210 towards second bay 220,or vise versus. According to another embodiment, some optical signalsinside optical interconnect apparatus 10 may travel from bay 210 to bay220, while some other optical signals may travel from bay 220 toward bay210 simultaneously.

By comparing with current optical signal interconnect arrangement, asbeing illustrated in FIGS. 10 and 11, where each piece of optical fiberprovides interconnect for only one optical signal of one wavelength,embodiment of present invention provides an optical interconnectapparatus that enables interconnect for multiple, for example, 4, 8, oreven more optical signals with only one optical signal path such as oneoptical fiber. The use of optical interconnect apparatus 10 provided byembodiment of present invention greatly reduces the number of individualpiece of fibers needed in the optical signal interconnect, illustratedin FIG. 2 by the intentionally missed use of “dots” as compared to thatin a conventional interconnected system shown in FIG. 10, saves costassociated with the fibers, increases the reliability control of opticalsystem 200, and eases the complexity in the event of troubleshooting.

According to one embodiment of present invention, optical interconnectapparatus 10 may be made sufficiently compact so as to fit into anincreasingly crowded space between bays that are interconnected, orwithin an individual bay within which there is normally not enough spaceto accommodate a large number of interconnecting fibers. For example,WDM filter 12 of optical interconnect apparatus 10 may be made bothcompact and light weight by using most recent technology includingthin-film based WDM filters, such as using WDM filter 130 moduledemonstratively illustrated in FIG. 8.

Reference is briefly made to FIG. 8, which illustrates a sample exampleof a WDM filter module that may be used in an optical interconnectapparatus or optical assembly, like optical interconnect apparatus 10 inFIG. 1, according to one embodiment of present invention. In FIG. 8,thin-film based WDM filter module 130 is demonstratively illustrated tohave an input/output port 133 with a built-in adaptor able to accept aninput/output connector, although input/output port 133 may be made in aform of pigtail fiber as well. A composite optical signal coming fromport 133 may be coupled into thin-film filter 136 via a collimator 137.Optical signals of different wavelengths may be coupled into a set ofcollimators 135 at one end, which has a set of corresponding pigtailfibers 131 attached thereto at their respective other end. Asemi-flexible protective sleeve 134 may be used to cover the pigtail endto provide additional cushion and flexibility to the set of pigtailfibers 131.

Thin-film filter 136 inside WDM filter module 130 may be made bystacking a plurality of individual single wavelength filters togetherthrough optical bonding, the making of which is described in moredetails in Applicant's Jun. 16, 2017 filed U.S. patent application Ser.No. 15/731,480, the content of which is hereby incorporated by referencein its entirety.

In making a WDM filter module such as WDM filter 12 in FIG. 1, accordingto one embodiment of present invention, the set of pigtail fibers 11 maybe compactly stacked together and attached to WDM filter 12. FIG. 9 is asimplified illustration of a cross-sectional view that demonstrates therelative size of the stack of pigtail fibers and that of the packagesize of WDM filter 12. According to one embodiment, in order toaccommodate tight space commonly found between interconnected bays, WDMfilter 12 may be made to have a cross-sectional size, L1×S1 as shown inFIG. 9, that is no larger than 10 times, preferably less than 7 timesand more preferably less than 4 times, the size of cross-section of thestack of pigtail fibers 11, L2×S2 as shown in FIG. 9. In one embodiment,as a non-limiting example, a WDM filter with four pigtail fibers may bemade to have L1 about 15 mm or less and S1 about 10 mm or less, while L2about 8 mm or less and S2 about 2 mm or less, making cross-section ofthe WDM filter less than 10 times of that of the stack of fibers, in adirection vertical to the stacking.

In another embodiment, WDM filter 12 and the set of pigtail fibers 11may together have a weight less than a predetermined amount such that,for example, when hanging from optical signal path 15, a total weight ofWDM filter 12 and associated set of pigtail fibers 11 may not exert atensile stress to optical signal path 15, which is usually a fiber, thatis measurable to cause distortion and/or delay of optical signal thatmay propagate inside optical signal path 15.

The above weight and size of optical interconnect apparatus 11, inparticular that of the WDM filter and associated pigtail fibers madeaccording to embodiment of present invention, enables the opticalinterconnect apparatus of present invention to fit into limited spaces.For example, optical interconnect apparatus 10 may simply hang over awall instead of being placed in a shelf to occupy a certain amount ofshelf space. In the meantime, the length of each pigtail fibers may bemade sufficiently long, but not excessively to save cost and space, toreach each connecting transponders in the shelfs.

Reference is made back to FIG. 3 and subsequent figures. FIGS. 3-7 aredemonstrative illustrations of optical interconnect apparatus or opticalassemblies with different input/output interfaces according toembodiment of present invention. For example, FIG. 3 demonstrativelyillustrates an optical assembly 30, or interconnect kit, that includes aset of pigtail fibers 31 coming out of a WDM filter 32. The other end ofWDM filter 32 may include an optical connector 33 of any suitable type,e.g., a LC-type connector. Optical assembly or interconnect kit 30 maybecome a part of optical interconnect apparatus 11 illustrated inFIG. 1. For example, first end 13 of optical signal path 15 of FIG. 1may be connected to WDM filter 32 via connector 33 through the use of asuitable adaptor.

Further for example, FIG. 4 demonstratively illustrates another opticalassembly 40, or interconnect kit, that is generally similar to opticalassembly 30 illustrated in FIG. 3, including having a set of pigtailfibers 41 coming out of WDM filter 42. On the other hand, opticalassembly 40 may have an optical connector 43 that is at the end of apigtail fiber pigtailed to WDM filter 42, other than being made as partof WDM filter 42 like connector 33 being directly attached to orextended from WDM filter 32 as in FIG. 3.

FIG. 5 demonstratively illustrates yet another optical assembly 50, orinterconnect kit, that is generally similar to optical assembly orinterconnect kit 30 in FIG. 3, including having a set of pigtail fibers51 coming out of a WDM filter 52. On the other hand, optical assembly 50may include a female adaptor 53 directly attached to WDM filter 52,instead of a male connector 33 directly attached to WDM filter 32 as inFIG. 3. Depending on the type of adaptors, female adaptor 53 maydirectly accept a male LC-type connector (or any other types ofconnectors) for interconnecting optical signals without going through anexternal adaptor, as it would be for connectors 33 and 43 in FIG. 3 andFIG. 4. Optical assembly 50 may also include a semi-flexible sleeve 54between the set of pigtail fibers 51 and WDM filter 52, similar tooptical assemblies 30 and 40 although not specifically described, whichprovides mechanical protection and ease of handling of optical assembly50.

FIG. 6 demonstratively illustrates an optical assembly 60, orinterconnect kit, that is similar to optical assembly 40 in FIG. 4,including having a set of pigtail fibers 61 coming out of WDM filter 62.Different from input/output port 43 in optical assembly 40 in FIG. 4,input/output port 63 of optical assembly 60 may be located on a sameside as that of the set of pigtail fibers 61. The flexibility oflocations of input/output port and the set of pigtail fibers furtherenhances the usability of optical assembly 60 in space-tightenvironment.

FIG. 7 demonstratively illustrates an optical assembly 70, orinterconnect kit, that is similar to optical assembly 50 in FIG. 5,including having a set of pigtail fibers 71 coming out of WDM filter 72.Different from female adaptor 53 in optical assembly 50 in FIG. 5, afemale adaptor 73 for input/output of optical assembly 70 may be locatedon a same side as that of the set of pigtail fibers 71 and is directlyattached to WDM filter 72. Therefore, similar to optical assembly 60where both the input/output port and the set of pigtail fibers are alsolocated on a same side of the optical assembly, the flexibility oflocations of input/output port (i.e., female adaptor 73) and the set ofpigtail fibers further enhances the usability of optical assembly 70 inspace-tight environment.

One or more optical assemblies or interconnect kits 30, 40, 50, 60, and70, as demonstratively illustrated above in FIGS. 3-7, may be used inconnection with optical signal path 15 of optical interconnect apparatus10 in providing alternative interconnect apparatus for optical signals.For example, first set of pigtail fibers 11 and first WDM filter 12(and/or second set of pigtail fibers 19 and second WDM filter 18) may bereplaced by one or more of above optical assemblies or interconnect kitsin optical interconnect apparatus 10.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the spirit ofthe invention.

What is claimed is:
 1. An optical interconnect apparatus comprising: anoptical signal path having a first and a second end and having only oneoptical path connecting the first and second ends; a firstwavelength-division-multiplexing (WDM) filter having a first end of twoor more input ports and a second end of a single output port; a secondWDM filter having a first end of a single input port and a second end oftwo or more output ports; and a first set of two or more pigtail fibersattached to the two or more input ports of the first end of the firstWDM filter, the single output port of the second end of the first WDMfilter attached to the first end of the optical signal path, the secondend of the optical signal path attached to the single input port of thefirst end of the second WDM filter, and the two or more output ports ofthe second end of the second WDM filter attached to a second set of twoor more pigtail fibers, wherein two or more optical signals are adaptedto propagate from the first set of two or more pigtail fibers into thefirst end of the first WDM filter, to propagate subsequently from thesecond end of the first WDM filter into the first end of the opticalsignal path, to propagate subsequently from the second end of theoptical signal path into the first end of the second WDM filter, and topropagate subsequently from the second end of the second WDM filter intothe second set of two or more pigtail fibers; wherein said a opticalsignal path is an optical fiber and wherein said first WDM filter and afirst set of fibers have a combined weight that is enabled by the use ofstacked thin-film filters such that, when the apparatus is hung fromsaid optical fiber, said combined weight is sufficiently light to notexert a tensile stress to cause measurable distortion and delay ofoptical signals that propagate inside said optical fiber.
 2. Theapparatus of claim 1, wherein at least one of said first and second endsof said optical signal path is connected to one of corresponding saidfirst and second WDM filters through an optical adaptor, and whereinsaid first WDM filter has a cross-sectional area that is less than tentimes a size of the first set of two or more pintail fibers beingstacked together, in a direction vertical to the stacking.
 3. Theapparatus of claim 1, wherein at least one of said first and second endsof said optical signal path is pigtailed from one of corresponding saidfirst and second WDM filters, wherein said first and second WDM filtershave wavelength-dependent functionalities that are matched to eachother.
 4. The apparatus of claim 1, wherein said first set of pigtailfibers comprises at least 4 individual pigtail fibers that are adaptedto accommodate four optical signals of four different wavelengthsrespectively, and wherein an optical signal of a first wavelengthpropagates from a corresponding first pigtail fiber to the first end ofthe first WDM filter; continues to the second end of the first WDMfilter; and continues to the first end of the optical signal path, andthe optical signal of the first wavelength experiences a total insertionloss of less than 0.5 dB from the first pigtail fiber to the first endof the optical signal path.
 5. The apparatus of claim 1, wherein saidfirst set of pigtail fibers are stacked together and mounted on one sideof said first WDM filter.
 6. The apparatus of claim 5, wherein saidfirst set of pigtail fibers and said first end of said optical signalpath are at a same side of said first WDM filter.
 7. An optical assemblycomprising: a wavelength-division-multiplexing (WDM) filter having afirst end of a set of input ports and a second end of a single outputport; and a set of pigtail fibers attached to said set of input ports ofsaid first end of the WDM filter, wherein the set of pigtail fibers arecompactly stacked together and mounted on one side of the WDM filterthat, in a direction vertical to said set of pigtail fibers, has across-sectional area less than ten times of a size of the stacked set ofpigtail fibers, and wherein a set of optical signals of differentwavelengths are adapted to travel from said set of pigtail fibers tosaid first end of said WDM filter via said set of input ports thereof,to travel subsequently to said second end of said WDM filter, and totravel subsequently to an optical signal path of a single fiber that isconnected to said single output port of said second end of said WDMfilter; wherein the WDM filter is a thin-film based WDM filter made of aplurality of compactly stacked individual single wavelength thin-filmfilters that creates a combined weight of the WDM filter and a set offour pigtail fibers that is sufficiently light, when the opticalassembly is hung from the optical signal path, to not exert a tensilestress to cause measurable distortion and delay of optical signals thatpropagate inside said optical signal/path.
 8. The optical assembly ofclaim 7, wherein said WDM filter further includes a single-width femaleoptical adaptor at said single output port for accommodating aconnectorized optical fiber to pass said set of optical signals ofdifferent wavelengths.
 9. The optical assembly of claim 7, wherein saidWDM filter further includes another pigtail fiber that is connectorizedand connected to, via an adaptor suitable for MT-RJ type connector, anoptical fiber of the optical signal path.
 10. The optical assembly ofclaim 7, wherein said WDM filter is a first WDM filter and said set ofpigtail fibers is a first set of pigtail fibers, further comprising: anoptical signal path with a first end thereof being attached to saidfirst set of pigtail fibers via said first WDM filter; a second WDMfilter; and a second set of pigtail fibers being attached to a secondend of said optical signal path via said second WDM filter, wherein atleast one of said first and second WDM filters comprises a set ofdifferent thin-film filters each of which being made for an opticalsignal of different wavelength.
 11. An interconnected optical systemcomprising: a first optical transport terminal having a first set ofoptical transponders with a first set of corresponding optical signalports; a second optical transport terminal having a second set ofoptical transponders with a second set of corresponding optical signalports; and an optical signal path connecting said first set of opticalsignal ports with said second set of optical signal ports, wherein saidfirst set of optical signal ports are connected to a first set ofpigtail fibers that are attached to a first end of a firstwavelength-division-multiplexing (WDM) filter, a second end of saidfirst WDM filter being connected to a first end of said optical signalpath; and wherein said second set of optical signal ports are connectedto a second set of pigtail fibers that are attached to a first end of asecond WDM filter, a second end of said second WDM filter beingconnected to a second end of said optical signal path, and wherein anoptical signal, from said first optical transport terminal, is adaptedto travel from said first set of pigtail fibers to said first end ofsaid first WDM filter, to travel subsequently to said second end of saidfirst WDM filter and subsequently to said first end of said opticalsignal path, to travel subsequently to said second end of said opticalsignal path and subsequently to said second end of said second WDMfilter, to travel subsequently to said first end of said second WDMfilter, to travel subsequently to said second set of pigtail fibers, andto travel and reach said second optical transport terminal; wherein atleast one of said first and second WDM filters is a thin-film based WDMfilter comprising a plurality of individual thin-film filters, each ofwhich filtering a different wavelength of optical signal, the pluralityof individual thin-film filters are compactly stacked one on top ofanother sequentially using optical bonding, the use of thin-film filtersand their sequential stacking enables the WDM filter to have across-sectional area of equal to or less than 15 mm-by-10-mm, a totaloptical/insertion loss of less than 0.5 dB, and a combined weight thatis sufficiently light to not exert tensile stress to a fiber, when beinghung from the fiber, that may cause measurable distortion and delay ofoptical signals that propagate inside said fiber.
 12. The optical systemof claim 11, wherein said first end of said optical signal path isconnected to said first WDM filter via a pigtail fiber or is connectedto said first WDM filter through a SC connector via an optical adaptorof single-width female type.
 13. The apparatus of claim 1, wherein thesecond WDM filter is a thin-film based WDM filter made of a plurality ofindividual thin-film filters each being made for filtering a differentwavelength, the thin-film based WDM filter receives a composite signalof the two or more optical signals and separates them into said secondset of two or more pigtail fibers, and wherein optical bonding is usedto stack together said individual thin-film filters one on top ofanother compactly.
 14. The apparatus of claim 1, wherein the first WDMfilter is a thin-film based WDM filter and wherein an optical signalpropagating from one of said first set of two or more pigtail fibers tosaid first end of said optical signal path through said thin-film basedWDM filter experiences less than 0.5 dB total insertion loss associatedwith said thin-film based WDM filter.
 15. The optical assembly of claim7, wherein the WDM filter is a thin-film based WDM filter comprising aplurality of individual thin-film filters each for filtering an opticalsignal of different wavelength, the individual thin-film filters arecompactly stacked together one on top of another through opticalbonding, the use of compactly stacked thin-film filters enables the WDMfilter, when having four pigtail fibers, to have a cross-sectional areaof equal to or less than 15 mm-by-10 mm.
 16. The optical assembly ofclaim 7, wherein the thin-film based WDM filter enables a totalinsertion loss of said set of optical signals less than 0.5 dB.
 17. Theoptical system of claim 11, wherein the second end of the first WDMfilter is connectorized and connected to, via an adaptor suitable forMT-RJ type connector, the first end of the optical signal path.